Thanks for the info in the past, but want to try something else. I know the check valve can be tested, but don't have acccess to my tools. When I turn off the motor and pull the check valve out of the booster ... I just about get sucked in ... Ok a little exageration. But quite a bit of suction there. If I turn the motor off and leave it over night, there is still some vacuum there, but far less. Question: How air tight should that be? Will it leak down any? Seems to me that I do have a bad check valve. PS- Anyone replace a vacuum booster with a GM brand, not aftermarket? The parts guy said it comes with a booster check valve. Reason I ask ... I had the dealer replace the booster and the check valve on it now is the original.